Generally, styrenic resins have good processability and mechanical properties and have accordingly been used to produce housing parts for many electrical and electronic goods. However, styrenic resins can be readily ignited and further can readily spread fire. Moreover, styrenic resins are subject to various mandatory controls on flammability for safety reasons in countries such as the United States, Japan and Europe, and are required to have high flame retardancy to meet the Underwriter's Laboratories Standard for use in the housings of electrical and electronic appliances.
A widely used and known method for imparting good flame retardancy to styrenic resin comprises adding a halogen-containing compound as a flame retardant to a rubber-modified styrenic resin and adding an antimony-containing compound as a flame retardant aid. Examples of halogen-containing compounds used to impart flame retardancy include polybromodiphenyl ether, tetrabromobisphenol-A, epoxy compounds substituted with bromine, chlorinated polyethylene, and the like. Antimony trioxide or antimony pentaoxide is commonly used as an antimony-containing compound.
When a halogen- and antimony-containing compound is used to improve flame retardancy of resins, a desired degree of flame retardancy can readily be imparted to the resulting products without significantly degrading the physical properties thereof. However, hydrogen halide gases released by halogen-containing compounds during processing can have fatal effects on the human body. Particularly, polybromodiphenyl ether, which is widely used as a halogen-containing flame retardant, may produce toxic gases such as dioxin or furan during combustion. Accordingly, there is a need to develop flame retardancy methods that do not employ halogen-containing compounds.
Rubber-modified styrenic resins generally have little remaining char during combustion, and thus it is hard to impart flame retardancy to such resins in their solid state (Journal of Applied Polymer Science, 1998, vol. 68, p. 106). Therefore, it may be necessary to add a char forming agent to a rubber-modified styrenic resin so that char can be formed in order to obtain desirable flame retardancy.
A well known and widely used method of imparting flame retardancy without using halogen-containing flame retardants uses a phosphate ester flame retardant. However, relatively high amounts of phosphate ester flame retardants or flame retardant aids are required to obtain sufficient flame retardancy, which can negatively impart other properties of the resin.
WO 2008/114858, WO 2008/108485 and US 2008/0227884 refer to 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, which is a phosphonate based cyclic compound, and a reactive flame retardant resin using the same. However, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is only used as a phosphorous-containing flame retardant hardener for imparting flame retardancy to thermoset (crosslinked and hardened) resins.